Platen drives have long been used in amusement parks and other environments where it is desirable to propel a vehicle along a track. A platen drive unit has two major components (1) a drive unit, which is positioned at or adjacent to the track, and which has a drive roller that frictionally engages and powers a passing vehicle, and (2) a platen, or flat, generally horizontal plate of the vehicle, that is mounted by the vehicle in a position to engage the drive roller and to receive therefrom propulsion sufficient for the vehicle to reach the next platen drive unit.
One well-known example of a platen drive system is the famous "PEOPLEMOVER" attraction, found at Disneyland Park.RTM. in Anaheim, Calif. In this attraction, a platen is positioned underneath each vehicle, in a horizontal plane. The underside of the platen is coated with a textured surface to increase friction. Thus, as the vehicle travels along a predefined path, the platen "runs over" individual drive rollers, and is powered thereby. Each platen drive unit is mounted at spaced intervals along the track, and the spacings of the intervals are configured such that the vehicle platen is always driven by at least one platen drive unit. This configuration enables a brake within each drive unit to freeze the vehicle, should an emergency condition occur on the track. Each drive unit typically has a high-voltage electric motor and gear reduction that slows down the motor speed to provide relatively slower, more powerful rotations of the drive roller to thereby propel each vehicle that passes the drive unit. Each platen drive unit can thus abstractly be described as a rotatably-mounted friction wheel which is driven by a gearmotor to propel a passing vehicle.
Platen drive schemes are frequently advantageous for propelling vehicles along a track, because each vehicle need not possess a drive unit, which imposes spatial constraints in vehicle design, requires a drive scheme that is more complex, increases vehicle weight, and requires a high voltage power supply to be placed in proximity to passengers riding the vehicle. Individual platen drive units along a track may be readily adjusted to provide different vehicle speeds at different positions of the track, without requiring a sophisticated control system or operator for each vehicle. In addition, the cost of running the attraction is fixed, and thus, additional vehicles may be added to the system without increasing operating costs. System maintenance is generally facilitated, since the platen drive units may be modularized, and easily replaced without requiring removal of the vehicle from service.
Also, platen drive systems are attractive from a speed-regulation standpoint, since if a platen approaches a drive unit at a speed which is less than the speed at which the drive roller is rotating, then the drive accelerates the vehicle. Conversely, if the platen is travelling faster than the roller rotation, then the drive brakes the vehicle speed.
In order to develop the friction needed to accelerate or brake the vehicle, a vertical contact force must be developed between the platen and the drive roller. This force is called the "normal force," and the manner in which the normal force is developed is of critical importance to the performance of a platen drive system. There are two basic schemes for developing the normal force which have been used in the prior art, including "fixed base" and "self energizing" schemes.
"Fixed base" drive units rely upon compression of the drive roller by the platen in order to develop the required normal force. These drive units use a friction wheel as the drive roller, such as a pneumatic tire, which is adjusted in its mounting until the top of the tire is substantially above the plane of the platen. Thus as the platen passes over the drive, the platen compresses the tire, thereby creating pressure against the platen and inducing a normal force. One particular advantage possessed by a fixed base drive unit is that it can provide both acceleration and braking force to vehicles of varying speeds that approach the drive unit. However, these drive units must also necessarily apply a greater magnitude of normal force than is necessary to propel the vehicles. This "stiffness" of the tire increases the bumpiness experienced by passengers of the vehicle.
A "self energizing" drive unit generally mounts the drive roller at the swinging end of a pivoting frame. This swinging end is held in position by a spring, such that the drive roller is forced to intersect a conveyance plane through which the platen of each vehicle will travel. The self energizing drives are particularly adapted to the fact that the platen of each vehicle will be mounted at a slightly different height. Consequently, the spring permits downward recoil of the swinging end of the frame upon engagement between the drive roller and the platen, and also ensures that the drive roller will engage the platen of each vehicle, provided that the spring is sufficiently stiff to position the drive roller against the platen. A related advantage is that the magnitude of the normal force applied to the platen is no larger than is absolutely necessary to develop the required friction force, thereby providing a smoother ride to vehicle passengers, relative to the fixed base drive units.
However, the known self energizing drives also have drawbacks. For example, any single drive can generally provide only one of acceleration or braking force. This drawback is occasioned by the drives' single, pivoting frame construction, wherein the drive roller can typically pivot away from a platen in only one direction. Very-stiff springs have been used in an attempt to provide both accelerating and braking force, although the stiffness of the springs detracts significantly from ride quality. In addition, while certain other spring-based self energizing drive units have been developed in an attempt to overcome this problem, for example, as shown in U.S. Pat. No. 3,530,800 to Watkins, additional problems are typically created by the use of a spring to directly bias the drive roller toward and through the conveyance plane.
For example, a drive roller is urged by these self energizing units (1) forcefully upward by a stiff spring, to maintain a high normal force with platens that engage with the roller, and thereby propel passing vehicles, and (2) toward and through the conveyance plane, to thereby contact the platens of each vehicle, which may have slightly varying heights within a defined tolerance with respect to the drive roller. As the platen contacts the drive unit, the top of the drive roller interferes with the plane of the platen, creating a bump or lurch felt by the passengers that is similar to that created by a fixed base drive. The interference ensures engagement as the spring presses the tire upward against the platen, but causes a sudden impact when the platen first engages the wheel. As a result, the ride quality of a spring-based self energizing drive is not much better than that of a fixed base drive.
Other types of self energizing platen drive units have been designed, in an attempt to overcome these problems, but are frequently too expensive. For example, drive rollers may be pneumatically or electromechanically actuated to engage a platen only at times when the platen is actually adjacent to the drive roller. However, the pneumatics or electromechanics that perform these functions require sensors and complicated control systems, which increase unit cost and maintenance requirements.
Another problem with the aforementioned drive units is the occurrence of excessive tire wear to drive units placed along curves in the track. This wear is caused by lateral movement of the platen relative to the drive unit as the vehicle turns while in continued engagement with the drive roller. As the vehicle moves through a curved portion of the track, the heading angle of the platen changes. When this heading angle is not aligned with the plane of rotation of the drive roller, "scrubbing" occurs, which causes excessive roller wear and energy consumption. In addition, undesired high lateral forces are applied to the motor and gearing that support the drive roller.
Thus, there exists a need for an improved platen drive unit that provides for adequate traction to propel vehicles, yet does not cause the vehicle to experience a strong "bump" or lurch as the drive unit engages the platen of the vehicle. In addition, there exists a need for a platen drive unit that provides adequate traction to propel vehicles, yet also is compliant for platens of various heights with respect to the drive roller and the general conveyance plane. Finally, a need exists for a platen drive unit that is compliant with lateral forces incurred by drive units positioned at curves. The combination of all of these features in a single platen drive unit would advantageously enable the use of a single drive unit, having a single footprint, to meet all drive unit needs, reducing inventory and maintenance requirements. The present invention satisfies these needs and provides further related advantages.